This invention relates generally to an optical fiber laser stabilizing apparatus and method thereof. More particularly, it relates to an apparatus for stabilizing a mode-locked optical fiber laser by feedbacking an optical signal to a laser resonator, and method thereof.
In most active type mode locking optical fiber laser, if stabilization is not realized, significant errors are caused in the optical signal. Generation of these errors makes the laser useless in the real system.
In order to solve these technical problems, the technology of stabilizing the optical fiber in the mode-locked optical fiber laser has been studied. So far, stabilization in the active type mode locking optical fiber laser has been studied in various ways.
The stabilization technology in the conventional active type mode locking optical fiber laser is to compensate for changes in the length of the laser resonator due to changes in the external environments. Thus, a signal processing method, etc., which monitors the optical signals using relaxation oscillation of the optical fiber or a 2xc3x972 light modulator in order to compensate for the changes in the length of the laser resonator, has been used.
As an example, U.S. Pat. No. 5,590,142 entitled xe2x80x9cMODE-LOCKED FIBER RING LASER STABILIZATIONxe2x80x9d, which was issued to Xuekang Shan, discloses the technology of stabilizing the length in the optical resonator to the point where the variations in the phase are minimized by measuring the variations in the phase of the laser output and the phase of the laser output light, in order to stabilize the laser output by measuring the changes in the phase of the laser output. Thus, there is advantageously disclosed that there is no need for a RF frequency driver because it uses lower frequency than the life of the gain medium to feedback the variation in the phase to the piezo-electric transducer (PZT).
However, the U.S. Pat. No. 5,590,142 has a drawback that the variations in the phase of the laser output and the synthesizer must be exactly measured.
Meanwhile, U.S. Pat. No. 5,646,774 entitled xe2x80x9cMODE-LOCKED LASER STABILIZATION METHOD AND APPARATUSxe2x80x9d, which was issued to Hidehiko Takara, etc., discloses the technology of stabilizing the laser resonator by measuring the relaxed oscillating frequency components from the laser resonator and then performing a signal processing for minimizing this signal, in order to stabilize the output of the laser resonator by measuring the relaxed oscillating frequency components. Thus, there is advantageously disclosed that the laser resonator could be stabilized using low frequency components of relaxed oscillating frequency components without an additional high frequency oscillator.
However, the U.S. Pat. No. 5,646,774 has the advantage that it does not need an additional high frequency generator, but it has a drawback it has to exactly measure the frequency components, as in the above-mentioned U.S. Pat. No. 5,590,142.
The present invention is contrived to solve the above conventional problems and is thus to stabilize the laser resonator simply, by detecting the reflected signal on the loop as an error signal to thus minimize the detected signal, not by detecting any optical signal or any specific frequency components to thus feedback it to the resonator.
In other words, an object of the present invention is to provide a mode-locked optical fiber laser stabilizing apparatus for stabilizing the length of the optical fiber laser resonator by feedbacking the optical signal to the laser resonator, in the direction where the signal reflected by the optical fiber loop mirror is detected as an error signal to minimize the signal, and method thereof.
In order to accomplish the above-mentioned purpose, a resonator stabilizing apparatus in an optical fiber laser according to the present invention includes a non-linear optical amplitude loop mirror (NALM) for non-linearly amplifying a laser pumped laser light, a linear loop mirror of a closed loop shape for confining the laser light sufficiently amplified by the non-linear optical amplitude loop mirror, a light isolator for proceeding the light confined by said linear loop mirror only in one direction, an optical coupler for detecting the light proceeding in the direction opposite to the light isolator by the linear loop mirror, the phase of which is varied in the non-linear optical amplitude loop mirror, and a piezo-electric transducer (PZT) for performing a signal processing for minimizing the amount of light proceeding in an opposite direction when the light detector detects the light proceeding in the opposite direction and feedbacking it again to stabilize the light, the light detector is coupled by the linear loop mirror and the optical coupler.
Also, a method of stabilizing a resonator having a loop mirror in an optical fiber laser according to the present invention is provided. The method includes the following steps. A first step is of non-linearly amplifying a laser pumped light and confining the amplified laser light in a closed loop route. A second step is of making said light confined in said closed loop route proceed only in one direction. A third step is of detecting any light proceeding in the direction opposite to the proceeding direction of said second step by said loop mirror when a phase of said light is varied. And a fourth step is of performing a signal processing in the direction where the amount of light in the opposite direction is minimized if the light proceeding in the opposite direction in the third step is detected and of feedbacking it again to stabilize the light.